Many years ago, in a college course on toxic plants, I learned about the popular ornamental houseplant dieffenbachia, also known as “dumb cane.” Its leaves contain microscopic crystals of calcium oxalate, and if someone innocently eats one of its thick, colorful leaves – even a tiny piece – they immediately suffer an extreme, stinging throat pain that prevents them from uttering a word. Hence the name dumb cane.
I thought this was just another interesting toxic plant story ... until I found out that many other plants also contain oxalate crystals, including some major forages and even some human foods. Whoa! What gives?
Textbooks on toxic plants feature many oxalate-containing species that live in the dry range country – plants such as halogeton (Halogeton glomeratus) and greasewood (Sarcobatus vermiculatus) – and also soursob (Oxalis pes-caprae) in Australia. Sheep and cattle grazing these plants show symptoms that range from labored breathing, weakness, tremors and depression, to convulsions and death. Thousands of sheep and cattle have died from oxalate poisoning, particularly in the arid regions of the American West, Australia and South Africa. Toxicity symptoms may also resemble hypocalcemia (a low blood calcium syndrome sometimes seen in ewes around lambing) because oxalates bind to calcium and can seriously lower the levels of serum calcium. Calcium oxalate crystals can also be deposited in the rumen wall and, importantly, in the kidney tubules, causing devastating kidney damage. A main cause of death from oxalate toxicity is kidney failure. And in horses, oxalates cause disturbances in the metabolism of calcium and phosphorus, resulting in lameness, misshapen bones and a syndrome called “bighead.”
Treatments? Not particularly good or successful. The basic recommendation is to move the affected animals away from these plants and give them supportive care for the symptoms. Provide lots of water to help the kidneys flush out the crystals. Maybe also give calcium intravenously to correct the low serum calcium levels. Unfortunately, once an animal shows symptoms, the kidney damage may be so great that the prognosis is poor.
One interesting wrinkle to this story occurs in ruminants such as sheep, cattle and goats. At least two species of rumen bacteria actually metabolize oxalates, so we can sometimes use strategic feed management to help ruminants adapt to these toxic plants, at least to some extent. Feeding small amounts or allowing very limited grazing of oxalate-containing plants can rapidly increase the rumen population of these bacteria so that the host animal becomes more tolerant of oxalates. The adaptation period may take as little as four days. This strategy, however, only works for ruminants. Horses cannot adapt unless they suddenly develop a rumen.
What are oxalates, and why do plants have them?
Plant oxalates are usually derived from oxalic acid, a very common molecule in the botanical world. Calcium combines with oxalic acid to form calcium oxalate, which precipitates in plant cells as calcium-oxalate crystals, generally in the leaves and stems. Oxalate crystals come in three distinct forms: raphides, styloids and druses. These may be great words for Scrabble, but they also have practical import.
These three forms are quite different from each other. Raphides are usually thin, needle-shaped crystals, sometimes in bundles, sometimes with barbs. Under a microscope, raphides look like ugly, menacing needles with hooks. You can see why they are not good things to get in your throat. Styloids come in a variety of shapes, such as prisms, including one shape with the great name “rhomboid.” Styloids usually occur as individual prismatic crystals inside cells. Druses crystals tend to be round or star-shaped, often in clusters, almost like a herd. Irritatingly, druses may also feature sharp points.
Oxalate crystals are not relegated just to toxic plants; they are actually found throughout the plant kingdom, in more than 200 families and 1,000 genera, from algae to broadleaf plants to grasses and even trees. And that elicits the obvious question: Why?
Research in this field is ongoing. Scientists have developed three possible reasons why plants accumulate oxalate crystals:
- Protection
- Storage
- Defense
Protection
Protection is easy to visualize – protection against something eating it. Our dumb cane is a good example. Oxalate crystals accumulate in its leaves. An animal comes along, grazes the leaves, feels great pain and avoids that plant in the future. Lesson learned. And, this doesn’t only apply to large herbivores, such as sheep, cattle and humans, but also to insects, slugs and other crawly things. However, this strategy doesn’t always work, especially when animals are very hungry. Those high death counts in arid regions are proof of that.
Storage
The second reason is storage. Oxalates can act as a plant’s bank account for calcium. Most plants are pretty good at absorbing calcium through their roots, but they don’t have good mechanisms for turning off that absorption process or excreting excess calcium. Plants don’t have kidneys. However, because oxalic acid is so readily available, plants can combine this compound with excess calcium to form calcium oxalates and then store the resulting crystals inside leaf cells. There is plenty of data to show that plants can mobilize calcium from the crystals when they need it. In the bigger picture, this storage strategy is similar to the way mammals utilize their bones as a storage organ for calcium. Clever species, these plants, and we should make no bones about it.
Defense
The third reason, defense, is intriguing and elegant. Like all other living organisms, plants are targets of infection, particularly by toxic fungi that can easily infect and destroy them. Plants don’t have lymph nodes or killer lymphocytes (T cells) or circulating antibodies, but they still must survive in a dangerous world. That’s where the oxalate crystals come in. Plant cells contain the enzyme oxalate oxidase, which splits calcium oxalate into carbon dioxide and hydrogen peroxide (in addition to releasing the calcium). This is the same hydrogen peroxide that you can buy in a drugstore. Recall how hydrogen peroxide hurts when you use it to disinfect minor cuts? Well, on a microscopic biochemical level, hydrogen peroxide is a powerful toxin against fungi. Hydrogen peroxide also stimulates nearby plant cells to increase the lignification in their cell walls, which strengthens the internal barriers against the further spread of fungi.
Potential nutritional value
Those three rationales – protection against herbivores, storage of excess calcium and defense against fungal invasion – help explain why oxalates are found in so many plants. In addition to the species mentioned earlier, other well-known plants can contain high levels of oxalates, such as kochia, redroot pigweed, curly dock, skunk cabbage and jack-in-the-pulpit. Many houseplants, such as dumb cane, contain high levels of oxalates, which means that we should not toss those plant cuttings into pastures.
Some forages also contain relatively high levels of oxalates. These include tropical and semitropical grasses such as elephant grass, guinea grass, African bristlegrass (Setaria sphacelata) and Kikuyu grass.
Also, alfalfa. Alfalfa? Yes, alfalfa. Although its oxalate levels are not high enough to be toxic to livestock, the oxalates do have a nutritional effect. Alfalfa often contains high levels of calcium, high enough to result in calcium-to-phosphorus ratios of 6-to-1 or higher. Over time, this imbalance could potentially cause serious bone problems, but in practice, it doesn’t. The reason is oxalates.
Some of this calcium, perhaps as much as 30%, is tied up in crystals of calcium oxalate. When animals eat alfalfa, these crystals flow through the gastrointestinal (GI) tract intact undigested, and their calcium atoms are not absorbed across the gut wall.
Oxalates are definitely involved in human nutrition. Most folks have heard about rhubarb. The stalks are delicious and safe, but the leaves are toxic. The reason? Oxalates. Other common human-food plants can contain significant levels of oxalates: spinach, beet leaves, Swiss chard, almonds, cashews and cocoa powder. There’s a lot of variation in the oxalate content of these plants, but in practice, the actual medical risks to humans are very low. Especially because, unlike livestock grazing in a fenced pasture, humans don’t usually consume diets composed of 100% spinach or 100% beet leaves. So, the real-world doses of oxalates in human diets are usually quite small and biologically insignificant.
Except when they’re not. We know that oxalate crystals cause kidney failure in livestock. These crystals can also accumulate in humans. Medical reports show that 70% to 80% of the cases of human kidney stones involve calcium oxalate crystals. Some individuals – due to diet, genetics, anatomy, family history, a previous kidney infection, etc. – are more predisposed to kidney stones than others. While kidney stone formation in humans is a complex and multifaceted medical problem, doctors routinely advise vulnerable patients to avoid eating oxalate-rich foods.
Oxalates and fodder beets
Which brings us back to forages. A new forage currently touted in the U.S. as the next best thing is fodder beets (Beta vulgaris), which are actually the same species as sugarbeets and Swiss chard. These are leafy plants grown and fed in many parts of the world, notably the U.K. and New Zealand. Fodder beets have been specifically bred for massive growth. An acre of fodder beets can yield more than 15 tons of dry matter. Very impressive. The large bulb is essentially a large ball of starch and sugar, and the leaves can provide a low-fiber, high-quality feed with crude protein levels of 12% to 15%.
But the leaves of fodder beets are ... beet leaves. One of the human foods that can contain high levels of oxalates is beet leaves. Hmmm. And, not surprisingly, the agricultural literature contains quite a few toxicity reports about animals that grazed fodder beets, especially in countries like New Zealand, where this crop is grown extensively. The reported symptoms include low blood calcium levels (milk fever) in dairy cows and hypocalcemia in ewes. Kidney stones and more than a few livestock deaths have also been reported. The culprit: oxalates.
So, what should we do? Fodder beets can be an excellent livestock feed. Huge tonnage, high quality. But perhaps we should use them gingerly, knowing their potential for accumulating oxalates. Fodder beets may be touted as the next best thing in the forage world, but we should still take them with a grain of salt.
